Candles, Yes...Soot, No
If you’ve noticed ghostly smudges and stains on your walls and ceilings, the culprit may be something you’d never suspect: candles you burn indoors.
Most candles produce soot, which can create stains on walls and ceilings. Just a few candles burned in a brand new home can leave soot deposits on surfaces.
The soot is easily suspended in the air. It deposits as a gray stain wherever air changes direction, like on horizontal surfaces, at picture frames, or under doors. Soot also deposits wherever the air slows because of cooling: at framing (where there is less insulation to the outdoors) and, in general, anywhere on exterior walls and ceilings. The result is ghostly stains outlining framing and fasteners.
Candles in glass jugs are the biggest soot generators. The jug limits air flow to the flame and this inhibits combustion, causing extra soot. I also have a sneaking suspicion that the composition of some candles (maybe the ingredients responsible for their sweet aroma) creates extra soot.
If you must burn candles – and who doesn’t like their sight and aroma? – buy higher-quality candles and keep wicks trimmed to about ¼ inch. This allows for a better-controlled flame and minimal soot production.